[GVN LoadPRE] Extend the scope of optimization by using context to prove safety of speculation

Use context to prove that load can be safely executed at a point where load is being hoisted.

Postpone the decision about safety of speculative load execution till the moment we know
where we hoist load and check safety at that context.

Reviewers: nikic, fhahn, mkazantsev, lebedev.ri, efriedma, reames
Reviewed By: reames, mkazantsev
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D88725